Mould for electroplating and its manufacturing process

ABSTRACT

A process for manufacturing a mould including: a) providing a first substrate made of photosensitive glass of thickness of at least equal to the height of the mould, b) illuminating the first substrate with UV rays through a mask the windows of which correspond to the depression of the mould in order to create illuminated zones, c) carrying out a heat treatment on the first substrate obtained in step b) in order to crystallize the illuminated zones, d) providing a second substrate having at least one conductive layer on its surface, e) joining the first substrate obtained in step c) with the second substrate so that the conductive layer is located between the first substrate and the second substrate, f) removing the illuminated and crystallized zones of the first substrate so as to uncover the conductive layer, forming a cavity with sidewalls and a bottom occupied by the conductive layer of the mould.

This application claims priority from European patent application No.17195237.7 filed on Oct. 6, 2017, the entire disclosure of which ishereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a mould intended to manufacture amicromechanical part by electroplating and to its manufacturing process.

BACKGROUND OF THE INVENTION

Electroplating has been used and known for a long time. LIGA processes(LIGA being the well-known abbreviation of the German expression“Röntgenlithographie, Galvanoformung & Abformung”) were developed in the80s and have proved to be very advantageous for manufacturinghigh-precision metal microstructures.

The principle of the LIGA technique consists in depositing a layer ofphotoresist on a conductive substrate or a substrate coated with aconductive layer; in carrying out, through a mask corresponding to theoutline of the desired microstructure, an x-ray irradiation by means ofa synchrotron; in developing, i.e. in removing via physical or chemicalmeans, the sections of the photoresist layer that were not exposed, inorder to define a mould having the outline of the microstructure; ingalvanically depositing a metal, typically nickel, in the photoresistmould; then in removing the mould in order to release themicrostructure.

The quality of the microstructures can hardly be criticized, but theneed to employ an expensive piece of equipment (synchrotron) makes thistechnique incompatible with mass production of microstructures that musthave a low unit cost.

For this reason analogous processes based on the LIGA process but usingphotoresists that are sensitive to UV have been developed. Such aprocess is for example described in the publication by A. B. Frazier etal, entitled “Metallic Microstructures Fabricated Using PhotosensitivePolyimide Electroplating Molds”, Journal of Microelectromechanicalsystems, Vol. 2, N deg. 2, June 1993, for the manufacture of metalstructures by electroplating metal in resist moulds made ofpolyimide-based photoresist. This process comprises the following steps:

-   -   providing a substrate including a conductive surface for seeding        a subsequent electroplating step,    -   applying to said conductive seeding surface a (polyimide)        photoresist layer,    -   irradiating the photoresist layer with UV through a mask        corresponding to the outline of the desired microstructure,    -   developing the portions of the photoresist layer that were not        irradiated by dissolving them so as to obtain a photoresist        mould,    -   galvanically depositing nickel in the open portion of the mould        up to the height of the latter,    -   separating the obtained metal structure from the substrate, and    -   removing the photoresist mould.

This process is widely used in the field of watch and clock making tomanufacture precision components. However, it proves to be the casethat, of the numerous timepiece components produced, many have a“spring” function, the spring working in the plane of the component orof the movement. The combination of the parameters “thickness of thecomponent—sought-after spring constant” may thus lead to very narrowspring-wire geometries (of a few tens of microns, or even less) forthicknesses of a several hundred microns.

Similarly, sometimes component design requires narrow slits to bedefined between two portions of the component, a mobile portion and afixed portion for example.

In these two cases, the conventional UV-LIGA technique reaches itslimits, both in terms of the aspect ratio (height/width ratio) of thegap to be filled with metal during the growth operation, and in terms ofthe aspect ratio of the resist separating two nearby geometries.

Furthermore, guaranteeing that the photoresist preserves its geometry(verticalness, size, etc.) in the galvanic bath is difficult, thishandicapping the robust manufacture of these specific timepiececomponents.

There is therefore a need for a process allowing such drawbacks to beovercome.

SUMMARY OF THE INVENTION

The aim of the present invention is to remedy the drawbacks associatedwith the conventional UV-LIGA process by providing an alternative mouldallowing the risks of deformation of the resist forming the mould usedin said conventional UV-LIGA process to be overcome.

Another aim of the invention is to provide a mould allowingmicromechanical parts having high-aspect-ratio geometries to be producedby electroforming.

To this end, the invention relates to a process for manufacturing amould comprising the following steps:

-   -   a) providing a first substrate made of photosensitive glass of        thickness of at least equal to the height of the mould    -   b) illuminating said first substrate with UV rays through a mask        the windows of which correspond to the depression of the mould        in order to create illuminated zones    -   c) carrying out a heat treatment on the first substrate obtained        in step b) in order to crystallize the illuminated zones    -   d) providing a second substrate having at least one conductive        layer on its surface    -   e) joining the first substrate obtained in step c) with the        second substrate so that the conductive layer is located between        the first substrate and the second substrate    -   f) removing the illuminated and crystallized zones of the first        substrate so as to uncover the conductive layer, in order to        form at least one cavity the sidewalls of which and the bottom        occupied by the conductive layer of which form said mould.

Such a method allows a mould made of glass, which is more rigid andinsensitive to the effects of the galvanic growth bath used to form amicromechanical part using said mould, to be produced.

The invention also relates to a process for manufacturing byelectroplating a metal micromechanical part, comprising the followingsteps:

-   -   g) manufacturing a mould of said micromechanical part using the        process for manufacturing a mould as defined above    -   h) filling the mould with a metal by galvanic growth from the        conductive layer in order to form said micromechanical part    -   i) releasing the micromechanical part obtained in step h) from        its mould.

The invention also relates to a multi-mould plate intended tomanufacture at least one micromechanical part by electroplating,including a first substrate made of photosensitive glass of thickness atleast equal to the height of the micromechanical part, a secondsubstrate securely fastened to the first substrate, at least oneconductive layer provided between the first substrate and secondsubstrate, the first substrate including at least one mould for themicromechanical part, which mould is formed by a cavity formed in saidfirst substrate and the bottom of which is occupied by the conductivelayer, allowing a metal to be deposited by galvanic growth in saidcavity in order to form said micromechanical part.

BRIEF DESCRIPTION OF THE DRAWINGS

Other particularities and advantages of the invention will become moreclearly apparent from the description that is given thereof below, byway of completely nonlimiting indication, with reference to the appendeddrawings, in which:

FIGS. 1 to 5 are representations of the successive steps of a processfor manufacturing a micromechanical part according to a first embodimentof the invention; and

FIGS. 6 to 11 are representations of successive steps of a process formanufacturing a micromechanical part according to a second embodiment ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 1 to 5, the present invention relates to aprocess for manufacturing a micromechanical part 1, 1′ byelectroplating. The process preferably includes a process formanufacturing a mould 3, 3′, followed by an electroplating step and astep of releasing the part 1, 1′ from said mould 3, 3′, respectively.

The process for manufacturing a mould 3, 3′ according to the inventionincludes successive steps intended to manufacture a mould 3, 3′:

A first step a) of the process for manufacturing the mould 3, 3′consists in providing a first substrate 5 made of photosensitive glassof thickness at least equal to the height of the mould.

Such a photosensitive glass is for example available from Schott A. G.under the trade name Foturan®, or from Hoya Corp. under the referencePEG3® or from LifeBioScience Inc. under the trade name Apex™.

Advantageously according to the invention, the use of photosensitiveglass makes it possible to generate in the glass a wide variety ofgeometries than etching silicon- or ceramic-based materials.

The second step b) of the process for manufacturing the mould 3, 3′consists in illuminating the first substrate 5 with UV rays, at awavelength corresponding to the photosensitive glass, through a maskthat is opaque at said wavelength and the windows of which correspond tothe depressions of the moulds to be produced, in order to createilluminated zones 7, 7′. Thus, depending on the amount, orientation anddistribution of the illumination, only those zones 7, 7′ of the firstsubstrate 5 which are exposed to the UV radiation are structured to formthe depressions of the moulds to be produced. The illumination sourcemay for example be a UV lamp the spectral distribution peak of which islocated between 200 and 400 nm.

The third step c) of the process of manufacturing the mould 3, 3′consists in carrying out a heat treatment on the first substrate 5obtained in step b) in order to crystallize the illuminated zones 7, 7′,as shown in FIG. 1. This heat treatment is carried out at hightemperature, preferably between 500° C. and 600° C. This heat treatmentallows the illuminated zones 7, 7′ to be made more selective with a viewto removal thereof in step f), as will be seen below.

The fourth step d) of the process for manufacturing the mould 3, 3′consists in providing a second substrate 8, said substrate comprising atleast one conductive layer 10 on its surface. Advantageously, theconductive layer is formed by depositing, on the second substrate 8, alayer 10 of a metal chosen from the group comprising chromium, titanium,and gold, gold being preferred. The conductive layer 10 preferably has athickness comprised between 0.1 μm and 0.5 μm.

The metal layer 10 has the double advantage of, on the one hand, beingconductive in order to allow the electroplating step h) to beimplemented and, on the other hand, of allowing the second substrate tobe joined to the first substrate by soldering in step e), as will bedescribed below.

The second substrate 8 is preferably made from a material that isresistant to acid attack. Advantageously, the second substrate 8 issilicon-based.

The fifth step e) of the process for manufacturing the mould 3, 3′consists in joining the first substrate 5 comprising the illuminated andcrystallized zones 7, 7′, i.e. the substrate such as obtained in stepe), to the second substrate 8 in such a way that the conductive layer 10is located between the first substrate 5 and the second substrate 8, asshown in FIG. 2.

In this first variant of the invention, the two substrates 5 and 8 arejoined by soldering by way of the conductive metal layer 10.

The sixth step f) of the process for manufacturing the mould 3, 3′consists in removing the zones 7, 7′ illuminated in step b) andcrystallized in step c) of the first substrate 5, so as to uncover theconductive layer 10 in order to form at least one cavity 12, 12′ thevertical sidewalls of which and the bottom occupied by the conductivelayer 10 of which form said mould 3, 3′, as shown in FIG. 3.

Advantageously, step f) of removing the illuminated and crystallizedzones 7, 7′ of the first substrate 5 is carried out by chemical etching,and preferably by dissolving with hydrofluoric acid. For example, thischemical etch may be carried out in an ultrasonic bath of about 10%hydrofluoric acid at room temperature.

A mould 3, 3′ the sidewalls of which are made of photosensitive glass isthus produced.

It will be noted that, contrary to conventional practice, the processaccording to the invention does not comprise a final higher-temperature(600-700° C.) crystallizing step for completely crystallizing theremaining photosensitive glass.

The present invention also relates to a process for manufacturing byelectroplating a metal micromechanical part 1, 1′. Said processcomprises a step g) that consists in manufacturing a mould 3, 3′ forsaid micromechanical part 1, 1′ using the process for manufacturing amould 3, 3′ described above. The following step h) consists in fillingthe mould 3, 3′ with a metal by galvanic growth from the conductivelayer 10 in order to form said micromechanical part 1, 1′, as shown inFIG. 4.

The metal is advantageously chosen from the group comprising nickel,copper, gold or silver, and the alloys thereof, such as gold-copper,nickel-cobalt, nickel-iron, and nickel-phosphorus.

Preferably, the height of the mould is slightly larger than the heightof the part to be manufactured and is equal to the thickness of thefirst substrate 5. The height of the micromechanical part is comprisedbetween 50 μm and 500 μm. The use of a substrate made of photosensitiveglass allows, depending on the thickness, parts to be obtained theminimum width of certain geometries of which may be comprised between 10μm and 30 μm, and hence such particular geometries of micromechanicalparts obtained using the process of the invention may have a high aspectratio comprised between 1 and 20. Such a micromechanical part 1, 1′could, for example, be a return spring, a jumper-spring, a single-pieceflexible guiding system, a compliant geometry for decreasing play, etc.

The electroforming conditions, in particular the composition of thebaths, the geometry of the system, the voltages and the currentdensities, are chosen for each metal or alloy to be electroplated usingtechniques that are well known in the art of electroforming (see forexample Di Bari G. A. “electroforming” Electroplating EngineeringHandbook 4th Edition, edited by L. J. Durney, published by Van NostrandReinhold Company Inc., N.Y., USA 1984).

It is possible, in a subsequent step h′), to rectify the metal depositelectroformed with the first substrate 5. This step may be carried outby grinding and polishing in order to directly obtain microstructureshaving a planar upper surface in particular having a surface finishcompatible with the requirements of the watch- and clock-making industryvis-à-vis the production of upmarket movements.

The following step i), which is illustrated in FIG. 5, consists inreleasing the micromechanical part 1, 1′ obtained in step h) or h′) fromits mould 3, 3′. To this end, chemical etching processes (for example HFprocesses for photosensitive glass, KOH processes for silicon) may beemployed.

The micromechanical part 1, 1′ thus released may be used directly orwhere appropriate after suitable machining.

FIGS. 6 to 11 show a second variant of implementation of the process formanufacturing a mould according to the invention. This process ispractically identical to the first variant described above, with theexception of a steps e) and f). The first substrate 5 comprising theilluminated and crystallized zones 7, 7′, as shown in FIG. 6, isobtained in steps a) to c) such as described above. In this secondvariant, the first substrate 5 and the second substrate 8 are joined instep e) by temporarily soldering the first substrate 5 to the secondsubstrate 8 by means of a resist layer 16 provided between said firstsubstrate 5 and the conductive layer 10, as shown in FIG. 7. To do this,the same type of resist as that used in semiconductor fabricationprocesses may be used for the temporary soldering steps. The resistlayer 16 in question has a thickness comprised between 2 μm and 10 μm.

Furthermore, in this second variant, the step f) comprises removing theilluminated and crystallized zones 7, 7′ of the first substrate 5 so asto uncover the resist layer 16 as shown in FIG. 8. Next, step f)comprises a substep f′) of removing the uncovered resist layer 16 fromthe bottom of the cavity 12, 12′ after the illuminated and crystallizedzones 7, 7′ of the first substrate 5 have been removed, so as to uncoverthe conductive layer 10, as shown in FIG. 9.

The micromechanical part 1, 1′ is then produced as in the electroplatingmanufacturing process described above, as shown in FIGS. 10 and 11. Thestep i) of releasing the part 1, 1′ from its mould 3, 3′ is carried outusing chemical etching processes (for example HF processes forphotosensitive glass, and KOH processes for silicon).

The processes according to the present invention allow micromechanicalparts having narrow and rigid geometries to be produced with a highprecision in batch mode.

Specifically, in a particularly advantageous manner, several moulds 3,3′ are produced in the same first substrate 5, 5′. These moulds are notnecessarily identical to one another. A multi-mould plate 14 is thenobtained as illustrated in FIG. 3.

This multi-mould plate 14 is intended to be used to manufacture at leastone micromechanical part 1, 1′ by electroplating. According to theinvention, it includes a first substrate 1, 1′ made of photosensitiveglass of thickness at least equal to the height of the micromechanicalpart 1, 1′, a second substrate 8 securely fastened to the firstsubstrate 5, at least one conductive layer 10 provided between the firstand second substrates 5, 8, and optionally a resist layer 16 providedbetween the first substrate 5 and the conductive layer 10. The firstsubstrate 5 includes at least one mould 3, 3′ for the micromechanicalpart 1, 1′, which mould is formed by a cavity 12, 12′ formed in saidfirst substrate 5 and the bottom of which is occupied by the conductivelayer 10, allowing a metal to be deposited in said cavity 12, 12′ bygalvanic growth with a view to forming said micromechanical part 1, 1′.The first and second substrates 5 and 8, the conductive layer 10 and theresist layer 16 are such as defined above.

In addition, the moulds obtained according to the invention are made ofphotosensitive glass, which is more rigid then the resistsconventionally used to form such moulds, and which is insensitive to theeffects of galvanic growth baths. The process for manufacturingmicromechanical parts according to the invention is thereforeparticularly robust, in particular for the manufacture ofhigh-aspect-ratio parts.

The processes according to the invention also allow the galvanic growthin step h) to be easily initiated by virtue of the use of a conductivelayer 10 of good quality at the bottom of the mould 3, 3′.

Furthermore, the process for manufacturing micromechanical partsaccording to the invention is simple to implement because it does notrequire a complex and localized deposition of metal (stencil mask) toform the layer used to initiate the galvanic growth.

Lastly, the process for manufacturing micromechanical parts according tothe invention allows geometries etched by Bosch® DRIE to be avoided.Hence scalloping does not occur, avoiding the need for an additionalsmoothing step.

1. A process for manufacturing a mould comprising the following steps:a) providing a first substrate made of photosensitive glass of thicknessof at least equal to the height of the mould b) illuminating said firstsubstrate with UV rays through a mask having windows which correspond tothe depression of the mould in order to create illuminated zones c)carrying out a heat treatment on the first substrate obtained in step b)in order to crystallize the illuminated zones d) providing a secondsubstrate having at least one conductive layer on its surface e) joiningthe first substrate obtained in step c) to the second substrate so thatthe conductive layer is located between the first substrate and thesecond substrate f) removing the illuminated and crystallized zones ofthe first substrate so as to uncover the conductive layer, in order toform at least one cavity the sidewalls of which and the bottom occupiedby the conductive layer of which form said mould.
 2. The processaccording to claim 1, wherein the second substrate is based on silicon.3. The process according to claim 1, wherein the conductive layer isformed by depositing, on the second substrate, a layer of a metal chosenfrom the group consisting of chromium, titanium and gold.
 4. The processaccording to claim 1, wherein the step e) of joining the first substrateand second substrate is carried out by soldering the conductive layer ofthe second substrate to the first substrate.
 5. The process according toclaim 1, wherein the step e) of joining the first substrate and secondsubstrate is carried out by soldering the first substrate to the secondsubstrate by means of a resist layer provided between the firstsubstrate and the conductive layer.
 6. The process according to claim 5,wherein step f) comprises a substep f′) of removing the resist layeruncovered in the cavity after removal of the illuminated andcrystallized zones of the first substrate, so as to uncover theconductive layer.
 7. The process according to claim 1, wherein the stepf) of removing the illuminated and crystallized zones of the firstsubstrate is carried out by chemical etching.
 8. The process accordingto claim 1, wherein a plurality of moulds are produced in the same firstsubstrate.
 9. A process for manufacturing by electroplating a metalmicromechanical part, wherein it comprises the following steps: g)manufacturing a mould of said micromechanical part using the process formanufacturing a mould according to claim 1 h) filling the mould with ametal by galvanic growth from the conductive layer in order to form saidmicromechanical part i) releasing the micromechanical part obtained instep h) from its mould.
 10. A multi-mould plate intended to manufactureat least one micromechanical part by electroplating, wherein it includesa first substrate made of photosensitive glass of thickness at leastequal to the height of the micromechanical part, a second substratesecurely fastened to the first substrate, at least one conductive layerprovided between the first substrate and second substrate, the firstsubstrate including at least one mould for the micromechanical part,which mould is formed by a cavity formed in said first substrate and thebottom of which is occupied by the conductive layer, allowing a metal tobe deposited by galvanic growth in said cavity in order to form saidmicromechanical part.
 11. The multi-mould plate according to claim 10,wherein the second substrate is based on silicon.
 12. The multi-mouldplate according to claim 10, wherein the conductive layer is a layer ofa metal chosen from the group consisting of chromium, titanium and gold.13. The multi-mould plate according to claim 10, wherein it comprises aresist layer provided between the first substrate and the conductivelayer.